When Sprint announced the winners of its "4G App Challenge" last week, you may have found yourself wondering what, exactly, constitutes a 4G app.

Looking at the winning mobile apps themselves there isn't much of a clear pattern as to what makes them especially 4G. From the Recipe Search reverse recipe lookup to the NASAImages app that lets users access thousands of space pictures to the mobile game "Say What?" that apes that classic children's game telephone, the winning apps all displayed a wide array of functionalities and no obvious signal of why they'd work especially well on a 4G network versus a 3G network.

"It's a bit more of an art than a science," says Teresa Kellett, Sprint's 4G director, when asked how Sprint determined the criteria for 4G applications. "There's not some special formula for what makes a 4G application. We try to look for apps that highlight some features of our 4G network, such as apps that take advantage of higher speeds or apps that specialize in transferring large files that you wouldn't want to download over a 3G network."

NASAImages is an example of an app that requires a big pipe for file transfers, as it gives users access to large, high-quality pictures. The "Say What?" game, meanwhile, is an app that requires reliable connectivity and low latency to be played properly and is thus best suited for a 4G network. And Jason Shah, the vice president of product management for app developer Mediafly, says that his company's video and radio streaming application was best optimized by sending it over a 4G network because of its high-quality video capabilities.

"In the initial testing for our app we'd be streaming at around 256Kbps," says Shah. "But we were able to bump that up to 512Kbps to 1Mbps on a 4G network. I mean, our app works fine over 3G or Wi-Fi but when it detects there's a 4G connection nearby it can switch to that and provide more high-quality videos and a less blocky experience."

In the end, then, a "4G app" isn't something that will reinvent the wheel but will rather operate mostly like a 3G app, only significantly faster and more reliable. Kellett says this is what she hopes app developers see in Sprint's WiMAX network and notes that Sprint and Clearwire's spectrum holdings will give mobile applications all the capacity they need.

"We're lucky to have a lot of spectrum," she says. "And what that translates to is the ability to have a better capacity and we're looking to support things like video apps that can take advantage of that capacity."